Add lock to RegisterVariantCollection

RELEASE-NOTES.md

@@ -44,6 +44,7 @@ END TEMPLATE-->
 
 ### Bugfixes
 
+* Fixed a race condition in parallel prototype loading that could corrupt the prototype variant collection and throw an `ArgumentNullException` when freezing prototype kinds.
 * Fixed a prototype losing its inherited data when a partial prototype changes its `parent:`.
 
 ### Other

Robust.Shared/Prototypes/PrototypeManager.Variantization.cs

@@ -85,21 +85,24 @@ private void ReplaceVariantNode(MappingDataNode parentNode, string variantNodeNa
     /// <param name="collectionVariants">A list of prototype variants derived from the same source prototype.</param>
     private void RegisterVariantCollection(KindData kindData, List<string> collectionVariants)
     {
-        if (kindData.UnfrozenVariants == null)
+        lock (_variantRegistrationLock)
         {
-            kindData.UnfrozenVariants = kindData.Variants.ToDictionary(kvp => kvp.Key, kvp => kvp.Value.ToList());
-        }
+            if (kindData.UnfrozenVariants == null)
+            {
+                kindData.UnfrozenVariants = kindData.Variants.ToDictionary(kvp => kvp.Key, kvp => kvp.Value.ToList());
+            }
 
-        kindData.UnfrozenVariants.EnsureCapacity(kindData.UnfrozenVariants.Count + collectionVariants.Count);
+            kindData.UnfrozenVariants.EnsureCapacity(kindData.UnfrozenVariants.Count + collectionVariants.Count);
 
-        foreach (var collectionMember in collectionVariants)
-        {
-            kindData.UnfrozenVariants[collectionMember] = collectionVariants;
+            foreach (var collectionMember in collectionVariants)
+            {
+                kindData.UnfrozenVariants[collectionMember] = collectionVariants;
+            }
         }
     }
 
     /// <summary>
-    /// Tries to get the list of all variants associated with a given prototype. 
+    /// Tries to get the list of all variants associated with a given prototype.
     /// </summary>
     /// <param name="collectionMember">The prototype being indexed.</param>
     /// <param name="collectionVariants">The collection of variants this prototype belongs to.</param>

Robust.Shared/Prototypes/PrototypeManager.cs

@@ -45,6 +45,11 @@ public abstract partial class PrototypeManager : IPrototypeManagerInternal
         private readonly Dictionary<string, Type> _kindNames = new();
         private readonly Dictionary<Type, int> _kindPriorities = new();
 
+        /// <summary>
+        /// Guards <see cref="KindData.UnfrozenVariants"/> against concurrent modification.
+        /// </summary>
+        private readonly Lock _variantRegistrationLock = new();
+
         protected ISawmill Sawmill = default!;
 
         private bool _initialized;